1,619 research outputs found
Principles of Control for Decoherence-Free Subsystems
Decoherence-Free Subsystems (DFS) are a powerful means of protecting quantum
information against noise with known symmetry properties. Although Hamiltonians
theoretically exist that can implement a universal set of logic gates on DFS
encoded qubits without ever leaving the protected subsystem, the natural
Hamiltonians that are available in specific implementations do not necessarily
have this property. Here we describe some of the principles that can be used in
such cases to operate on encoded qubits without losing the protection offered
by the DFS. In particular, we show how dynamical decoupling can be used to
control decoherence during the unavoidable excursions outside of the DFS. By
means of cumulant expansions, we show how the fidelity of quantum gates
implemented by this method on a simple two-physical-qubit DFS depends on the
correlation time of the noise responsible for decoherence. We further show by
means of numerical simulations how our previously introduced "strongly
modulating pulses" for NMR quantum information processing can permit
high-fidelity operations on multiple DFS encoded qubits in practice, provided
that the rate at which the system can be modulated is fast compared to the
correlation time of the noise. The principles thereby illustrated are expected
to be broadly applicable to many implementations of quantum information
processors based on DFS encoded qubits.Comment: 12 pages, 7 figure
A Note on the correspondence between Qubit Quantum Operations and Special Relativity
We exploit a well-known isomorphism between complex hermitian
matrices and , which yields a convenient real vector
representation of qubit states. Because these do not need to be normalized we
find that they map onto a Minkowskian future cone in , whose
vertical cross-sections are nothing but Bloch spheres. Pure states are
represented by light-like vectors, unitary operations correspond to special
orthogonal transforms about the axis of the cone, positive operations
correspond to pure Lorentz boosts. We formalize the equivalence between the
generalized measurement formalism on qubit states and the Lorentz
transformations of special relativity, or more precisely elements of the
restricted Lorentz group together with future-directed null boosts. The note
ends with a discussion of the equivalence and some of its possible
consequences.Comment: 6 pages, revtex, v3: revised discussio
Experimental Implementation of Logical Bell State Encoding
Liquid phase NMR is a general purpose test-bed for developing methods of
coherent control relevant to quantum information processing. Here we extend
these studies to the coherent control of logical qubits and in particular to
the unitary gates necessary to create entanglement between logical qubits. We
report an experimental implementation of a conditional logical gate between two
logical qubits that are each in decoherence free subspaces that protect the
quantum information from fully correlated dephasing.Comment: 9 Pages, 5 Figure
A Method for Modeling Decoherence on a Quantum Information Processor
We develop and implement a method for modeling decoherence processes on an
N-dimensional quantum system that requires only an -dimensional quantum
environment and random classical fields. This model offers the advantage that
it may be implemented on small quantum information processors in order to
explore the intermediate regime between semiclassical and fully quantum models.
We consider in particular system-environment couplings which
induce coherence (phase) damping, though the model is directly extendable to
other coupling Hamiltonians. Effective, irreversible phase-damping of the
system is obtained by applying an additional stochastic Hamiltonian on the
environment alone, periodically redressing it and thereby irreversibliy
randomizing the system phase information that has leaked into the environment
as a result of the coupling. This model is exactly solvable in the case of
phase-damping, and we use this solution to describe the model's behavior in
some limiting cases. In the limit of small stochastic phase kicks the system's
coherence decays exponentially at a rate which increases linearly with the kick
frequency. In the case of strong kicks we observe an effective decoupling of
the system from the environment. We present a detailed implementation of the
method on an nuclear magnetic resonance quantum information processor.Comment: 12 pages, 9 figure
SOPHIE velocimetry of Kepler transit candidates. XV. KOI-614b, KOI-206b, and KOI-680b: a massive warm Jupiter orbiting a G0 metallic dwarf and two highly inflated planets with a distant companion around evolved F-type stars
We report the validation and characterization of three new transiting
exoplanets using SOPHIE radial velocities: KOI-614b, KOI-206b, and KOI-680b.
KOI-614b has a mass of and a radius of
, and it orbits a G0, metallic
([Fe/H]=) dwarf in 12.9 days. Its mass and radius are familiar and
compatible with standard planetary evolution models, so it is one of the few
known transiting planets in this mass range to have an orbital period over ten
days. With an equilibrium temperature of K, this places
KOI-614b at the transition between what is usually referred to as "hot" and
"warm" Jupiters. KOI-206b has a mass of and a
radius of , and it orbits a slightly evolved F7-type
star in a 5.3-day orbit. It is a massive inflated hot Jupiter that is
particularly challenging for planetary models because it requires unusually
large amounts of additional dissipated energy in the planet. On the other hand,
KOI-680b has a much lower mass of and requires less
extra-dissipation to explain its uncommonly large radius of . It is one of the biggest transiting planets characterized so far,
and it orbits a subgiant F9-star well on its way to the red giant stage, with
an orbital period of 8.6 days. With host stars of masses of
and , respectively, KOI-206b,
and KOI-680b are interesting objects for theories of formation and survival of
short-period planets around stars more massive than the Sun. For those two
targets, we also find signs of a possible distant additional companion in the
system
Artificial Neural Networks for Classification in Metabolomic Studies of Whole Cells Using 1H Nuclear Magnetic Resonance
We report the successful classification, by artificial neural networks (ANNs), of 1H NMR spectroscopic data recorded on whole-cell culture samples of four different lung carcinoma cell lines, which display different drug resistance patterns. The robustness of the approach was demonstrated by its ability to classify the cell line correctly in 100% of cases, despite the demonstrated presence of operator-induced sources of variation, and irrespective of which spectra are used for training and for validation. The study demonstrates the potential of ANN for lung carcinoma classification in realistic situations
Characterization of the four new transiting planets KOI-188b, KOI-195b, KOI-192b, and KOI-830b
The characterization of four new transiting extrasolar planets is presented
here. KOI-188b and KOI-195b are bloated hot Saturns, with orbital periods of
3.8 and 3.2 days, and masses of 0.25 and 0.34 M_Jup. They are located in the
low-mass range of known transiting, giant planets. KOI-192b has a similar mass
(0.29 M_Jup) but a longer orbital period of 10.3 days. This places it in a
domain where only a few planets are known. KOI-830b, finally, with a mass of
1.27 M_Jup and a period of 3.5 days, is a typical hot Jupiter. The four planets
have radii of 0.98, 1.09, 1.2, and 1.08 R_Jup, respectively. We detected no
significant eccentricity in any of the systems, while the accuracy of our data
does not rule out possible moderate eccentricities. The four objects were first
identified by the Kepler Team as promising candidates from the photometry of
the Kepler satellite. We establish here their planetary nature thanks to the
radial velocity follow-up we secured with the HARPS-N spectrograph at the
Telescopio Nazionale Galileo. The combined analyses of the datasets allow us to
fully characterize the four planetary systems. These new objects increase the
number of well-characterized exoplanets for statistics, and provide new targets
for individual follow-up studies. The pre-screening we performed with the
SOPHIE spectrograph at the Observatoire de Haute-Provence as part of that study
also allowed us to conclude that a fifth candidate, KOI-219.01, is not a planet
but is instead a false positive.Comment: 13 pages, 4 figures, 6 tables, final version accepted for publication
in A&
A Study of Quantum Error Correction by Geometric Algebra and Liquid-State NMR Spectroscopy
Quantum error correcting codes enable the information contained in a quantum
state to be protected from decoherence due to external perturbations. Applied
to NMR, quantum coding does not alter normal relaxation, but rather converts
the state of a ``data'' spin into multiple quantum coherences involving
additional ancilla spins. These multiple quantum coherences relax at differing
rates, thus permitting the original state of the data to be approximately
reconstructed by mixing them together in an appropriate fashion. This paper
describes the operation of a simple, three-bit quantum code in the product
operator formalism, and uses geometric algebra methods to obtain the
error-corrected decay curve in the presence of arbitrary correlations in the
external random fields. These predictions are confirmed in both the totally
correlated and uncorrelated cases by liquid-state NMR experiments on
13C-labeled alanine, using gradient-diffusion methods to implement these
idealized decoherence models. Quantum error correction in weakly polarized
systems requires that the ancilla spins be prepared in a pseudo-pure state
relative to the data spin, which entails a loss of signal that exceeds any
potential gain through error correction. Nevertheless, this study shows that
quantum coding can be used to validate theoretical decoherence mechanisms, and
to provide detailed information on correlations in the underlying NMR
relaxation dynamics.Comment: 33 pages plus 6 figures, LaTeX article class with amsmath & graphicx
package
SOPHIE velocimetry of Kepler transit candidates XI. Kepler-412 system: probing the properties of a new inflated hot Jupiter
We confirm the planetary nature of Kepler-412b, listed as planet candidate
KOI-202 in the Kepler catalog, thanks to our radial velocity follow-up program
of Kepler-released planet candidates, which is on going with the SOPHIE
spectrograph. We performed a complete analysis of the system by combining the
Kepler observations from Q1 to Q15, to ground-based spectroscopic observations
that allowed us to derive radial velocity measurements, together with the host
star parameters and properties. We also analyzed the light curve to derive the
star's rotation period and the phase function of the planet, including the
secondary eclipse. We found the planet has a mass of 0.939 0.085
M and a radius of 1.325 0.043 R which makes it a member
of the bloated giant subgroup. It orbits its G3 V host star in 1.72 days. The
system has an isochronal age of 5.1 Gyr, consistent with its moderate stellar
activity as observed in the Kepler light curve and the rotation of the star of
17.2 1.6 days. From the detected secondary, we derived the day side
temperature as a function of the geometric albedo and estimated the geometrical
albedo, Ag, is in the range 0.094 to 0.013. The measured night side flux
corresponds to a night side brightness temperature of 2154 83 K, much
greater than what is expected for a planet with homogeneous heat
redistribution. From the comparison to star and planet evolution models, we
found that dissipation should operate in the deep interior of the planet. This
modeling also shows that despite its inflated radius, the planet presents a
noticeable amount of heavy elements, which accounts for a mass fraction of 0.11
0.04.Comment: 11 pages, 9 figure
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